In wireless communication systems, an air interface is used for the exchange of information between a mobile station and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels. In wireless transmission, a channel is time varying due to fading, mobility, and so on. More specifically, channel quality is affected by factors such as distance between the mobile and base station, speed of the mobile station, interference, and the like. Given the limited resources (e.g., bandwidth) of wireless transmission as well as the large number of mobile stations supported by a base station at any given time and therefore competing for those limited resources, it is therefore important to maximize throughput of a wireless communication system. For example, in a time-multiplexed system in which the transmission time interval spans one or more time slots, system throughput can be maximized by allowing a user with the best channel quality to transmit ahead of users with comparatively low channel quality.
In one known arrangement, a mobile station performs a rate calculation based on measurements of a pilot signal from the base station once every time slot and then reports back the rate at which it is going to receive data from the base station. Alternatively, the mobile station can send channel quality feedback information to the base station and the base station can then select the appropriate rate corresponding to that channel quality. In general, the purpose of sending channel quality feedback information from the mobile station to the base station is to inform the base station of the transmission rate that best matches the current conditions (e.g., quality) of the channel at that present time.
FIG. 1 shows one example of wireless transmission based on the well-known 1x-EV-DO (data only) standard in which a so-called fast rate adaptation scheme is used to maximize the system throughput by exploiting the time varying channel conditions. More specifically, FIG. 1 shows a signaling diagram between a base station (BS) and a mobile station (MS). The data channel 100 for the downlink from the base station to the mobile station is divided into time slots 101-117, each of which has a duration τ. In the case of the 1x-EV-DO standard, the duration τ would be 1.67 milliseconds. In the example shown in FIG. 1, time slots 101-106 and 110-115 are not carrying transmissions from the base station to the mobile station. However, the base station is transmitting to the mobile station during time slots 107-109 and during time slots 116-117.
From the perspective of the mobile station, the uplink channel 200 is also divided into time slots 201-217, each of which has a duration τ. As shown in FIG. 1, the calculation and reporting of the rate for the dedicated control channel 200 by the mobile station is performed according to well-known methods once every time slot 201-217. More specifically, channel quality information (e.g., transmission rate in 1x-EV-DO systems) is transmitted via communications 151-165. For example, mobile station is transmitting channel quality information to the base station via communications 151-153 during time slots 201-203 indicating that rate R1 is the desired rate of transmission based on the channel quality. As shown in this example, this rate information is received at the base station during time slots 103-105, so there is some delay with respect to the calculation of the rate in the mobile station during a time slot and the subsequent reporting of that rate to the base station. In slot 204, the mobile station detected a change in channel quality and reported back to the base station that transmission should occur at rate R2. This channel quality feedback is received by the base station during its time slot 106. Because the base station is sending a transmission to the mobile station during time slots 107-109 and because the base station is receiving channel quality feedback indicating rate R2 as the desired rate (e.g., R2 reported by mobile station during its time slots 204-206 and received by the base station during its time slots 106-108), the transmission by the base station during time slots 107-109 is therefore sent at rate R2. In time slots 207-212, the mobile station is reporting rate R1 as the desired rate, but no transmissions are occurring from the base station during time slots 110-115. As shown, the mobile station reports rate R3 as the desired rate during time slots 213-216, which are received at the base station starting at time slot 115. Because the base station starts sending another transmission during time slot 116, this transmission therefore is sent at rate R3 based on the channel quality feedback received from the mobile station.
Another approach to providing rate feedback is described in U.S. patent application Ser. No. 09/716,106, entitled “Asymmetric Rate Feedback and Adjustment System for Wireless Communications” and filed on Nov. 17, 2000, the subject matter of which is incorporated herein by reference in its entirety. As described, rate calculation and prediction is still being performed at every time slot, but the reporting of this rate feedback from a mobile station to the base station occurs every third time slot. In this manner, the reporting of rate feedback from a plurality of mobile stations can be staggered such that each mobile station is only sending rate feedback every third time slot, but the base station is receiving rate feedback at each time slot (i.e., from different mobile stations).
There are several disadvantages to these constant rate feedback approaches. Performing rate calculation and prediction in every time slot uses up a large amount of processing overhead at the mobile station. Providing rate feedback to the base station at every time slot uses a large amount of transmission overhead and, because rate feedback is being provided regardless of whether there is a transmission being sent from the base station, transmission resources are therefore being inefficiently used. Even if rate feedback is provided at a slower rate (e.g., every third time slot), there is still inefficient use of resources because the rate feedback is being provided regardless of whether the base station is transmitting.